In general, RFID systems consist of a tag or multiplicity of tags implemented to provide information such as identity, features, or characteristics of an object or article to which the tag is affixed, and to transmit that information via an RF signal to a RFID reader in response to an RF interrogation signal received by the tag from the reader. In many instances of supply chain tagging applications the tag may be placed on a container (e.g., a carton, a case or a pallet) utilized for holding or storing a multiplicity of common items. In contrast, item-level RFID tagging involves outfitting each individual item with its own RFID tag. The identity of and perhaps other information relating to the tagged article is stored in the tag's memory for transmission to a remote RFID reader, sometimes referred to as an interrogator, in response to a scan (or query, command or interrogation—these terms, for present purposes, meaning substantially the same thing) from the reader when within the response range of the tag.
The response range is any range suitable for enabling an RF communication session to take place between reader and tag. Thus, although the term radio frequency identification, or RFID, may tend to connote a one-way transfer of information from a tagged object to a remote location to reveal the object's ID, RFID systems typically involve two-way communication that is prompted by the reader's scan of the tag with either a continuous wave (CW), usually to evoke a read-only (RO) response by way of acquiring specific data stored in a specific format in the tag, or a modulated (generally amplitude modulated, or AM) RF signal to engage in a read/write (R/W) communication session with the tag.
In its simplest form, a conventional RFID tag consists of an application specific integrated circuit (ASIC) and an antenna. Often, the RFID tag is referred to as a transponder. These small, simple tags are in use in a variety of applications beyond supply-chain tagging, such as tracking movable assets, e.g., as diverse as rail cars and locomotives to cattle and other animals, or by way of on-the-fly collection of tolls assessed upon passage of a moving vehicle equipped with an authorized tag through a highway toll plaza. Another RFID tag application may involve control of access to secure areas of a facility or to entertainment events. A host of other applications may be served where moderate communication distances and moderate data transfer are required, notwithstanding the possible nearby presence of RFID readers other than merely the reader with which an ongoing communication session is intended.
RFID tags may be either passive or active. A passive RFID tag lacks an internal self-sufficient power supply, e.g., a battery, and relies instead on the incoming RF query by the reader to produce sufficient power in the tag's internal circuitry to enable the tag to transmit a response. In essence, the query induces a small electrical current in the tag's antenna circuitry, which serves as the power source that enables tag operation. Typically, the range of passive tags is more limited than that of active tags.
But the absence of a battery leads to certain advantages, primarily that a passive tag can have virtually unlimited life and be fabricated at much less cost and in considerably smaller size than an active tag, thus serving an important need to improve the efficiency and accuracy of tracking systems for commerce, security and defense. With costs of production trending downward, passive RFID tags could soon replace the ubiquitous universal product code (UPC) for many applications, the bar code strip found on myriad products and product containers in the stream of commerce. Unlike RFID, the imprinted bar code strip requires a line of sight optical scan to produce the UPC readout and the resulting computerized display or printout of price (at a point of sale of the bar-coded product) and other information regarding the product.
Although its on-board battery can provide the active RFID tag with greater dynamic range, higher data rates and additional functions that require a constant supply of power vis-à-vis the passive tag, and the battery itself may be quite small, the disadvantages of limited life, greater cost and size relative to the passive tag may weigh against the use of an active tag in certain applications.
The principles of the present invention are applicable to both passive and active RFID tags.
RFID tags may operate as read-only devices, capable of transmitting only fixed, invariable information stored in the tag memory of the semiconductor integrated circuit (IC) chip in which the tag is fabricated, as the readout when the tag is scanned by the reader in an RF communication between reader and tag. RFID tags may also be readable/writable devices adapted to allow their memories to be read and/or overwritten by a reader during a communication session. Data stored in memory (e.g., electrically-erasable programmable read-only memory, or EEPROM), whether original, overwritten or new, is available for transmittal to the reader on receipt by the read/write RFID tag of an appropriate command. Tag memory may contain both a read only portion and a read/write portion.
The form of communication known as modulated backscatter typically used by passive tags is a decades-old technique. Tags that communicate in this way can be very low power, with operational distances as great as tens of meters for radio signals in the ultra high frequency (UHF) or microwave bands.
Generally, the RFID tag is implemented in an ASIC chip fabricated, for example, using CMOS (complementary metal oxide silicon) process technology. The tag's electronics are integrated into the chip, sometimes referred to as a radio frequency IC, or RFIC, together with an antenna on the same substrate. The custom IC of which the RFIC or ASIC is comprised may include a voltage-doubler, analog and digital circuitry of the transponder, and memory capacity to store a software program as well as data to be transmitted to the reader in response to a command, plus other electronics as may be necessary for a particular RFID design.
A typical conventional RFID tag reader employs a transceiver, a control unit and an antenna for communicating with (e.g., interrogating or commanding) the tag at a designated RF frequency among several allocated for this purpose.
In the specific example of a RFID system in which a reader (interrogator) and a tag (transponder) are used to complete transactions in moderate to high speed applications such as toll collection transactions by the aforementioned identification of tagged authorized vehicles passing the reader in designated lanes of a toll plaza at or near highway speed (i.e. non-stop), the system may be impacted by the presence of multiple readers.
RFID tag ASICs operate with fixed RF thresholds that must be surpassed by the incoming RF signal (incident on the RFID tag's antenna) in order to present the tag's digital logic with an appropriate control bit, either a “1” or a “0.” This fixed threshold setting effectively limits rejection of RF interference encountered by the tag in an application where multiple readers may be present or in the vicinity of the tag, to a specific RF level. In essence, the limited rejection prevents the tag's data detector circuit from detecting a low RF level from a reader with which an RF communication session is intended to take place, if an interfering source (e.g. another reader's RF transmission) has exceeded the fixed threshold setting.
For example, RFID systems toll collection applications require an operating zone of 10 feet or more in the traveling direction, and exceeding the width of a roadway lane. This operating zone, often referred to as the “footprint” for reliable on-the-fly toll collection, is a range for RF communications to take place between reader and tag, outside of which communications may be spotty or unreliable. The footprint is one of the determining factors in a RFID system toward attaining reliable multiple transactions between a reader and a tag or tags within that zone.
In this footprint, the RF level increases by 20 db or more from a wake-up level, which is the RF level at which the tag first starts to respond. If the interference level reaches the fixed threshold anywhere in the footprint, the tag is no longer able to decode the reader's commands. It is not unusual for the interference to increase as the tag moves closer to the reader in a multi-lane configuration, such as at a toll plaza with two or more lanes devoted to on-the-fly toll collection of tagged vehicles, since overlapping antennas and readers are present to provide all toll lanes with adequate coverage. Consequently, readers in adjacent lanes act as sources of interfering RF transmissions depending upon the lane in which the tag is located.
FIG. 1 is a block diagram of the receiver input section of a typical prior art RFID tag. The RF signal transmitted by a reader encountered by the tag is incident on the tag's antenna 12. The RF input is an on-off signal, depicted as “RF in” in FIG. 2A, one in a sequence of waveform diagrams of FIG. 2 illustrating operation of the receiver input section of the RFID tag of FIG. 1. It will be observed that the RF in signal is relatively clean, which is the ideal case where noise or other signal interference is absent at the tag's antenna 12. The “RF in” signal is applied to an RF Detector 15, which converts that on-off RF signal to an analog baseband signal, depicted as “RF Detector out” in FIG. 2B. In fact, the latter signal may (and typically would) include an on-off component of variable amplitude and also interference and other readers' signals. Hence, despite its appearance in FIG. 2B, it is analog, not a true digital signal.
The “RF detector out” signal is fed into a Data Detector, or Comparator 17, which receives as a second input a fixed threshold level (FIG. 2C), referred to as the Trip Point, from a Threshold Generator 19. Thus, the Data Detector 17 is presented with the two inputs consisting of the baseband signal (“RF Detector out”) and the fixed threshold level (Trip Point) as shown in FIG. 2D, for comparison. When the baseband signal level exceeds the fixed threshold level, the output of Data Detector 17 is asserted and results in generation of the digital data depicted as “Comparator out” in FIG. 2E.
In situations when noise or other RF interference is present at antenna 12 along with the RF signal from an authorized RFID reader seeking to enter into a communication session with the tag, however, the resulting output of Data Detector 17 may be corrupted to an extent that precludes entry into the desired session. And this may be the result even where the noise or other signal interference is at a relatively low level. That type of situation is illustrated in the sequence of waveform diagrams of FIGS. 3A-3D. Here, the “RF in” signal (FIG. 3A) at the antenna 12 (FIG. 1) has a noise component 20. Accordingly, the output signal (“RF Detector out,” FIG. 3B) of RF Detector 15 resulting from the conversion of “RF in” also shows a presence of noise or interference spikes 22. To the extent that any of these spikes 22 exceed the fixed threshold level 25 (FIG. 3C), in the comparison of levels of the two signals by Data Detector 17, the digital output of the Data Detector is similarly fraught with errors, as illustrated by the inappropriate digital peaks 26 in the waveform diagram of FIG. 3D that are coincident with the excessive spikes.
It is essential that RFID systems operate with great accuracy and reliability, particularly in applications where transactions between RFID readers and tags are occurring at high speed. Any problems that may occur in reading the tags that participate in such transactions, as in an on-the-fly toll collection application, for example, directly affect the Toll Authority's revenue, making it imperative to resolve such problems in a manner that will result in reliable and consistent performance by the system.